The present invention relates to a function selector circuit for selecting an operation function indicated in a truth table in accordance with a combination of a plurality of input signal states, particularly to a function selector circuit used with a dual port video RAM having a number of operation functions.
As is often the case for a memory having a number of operation functions such as a dual port video RAM (hereinafter called DPRAM) used in recording and processing image data, a truth table for defining the correspondence between a plurality of external input signal states and a plurality of operation functions is used differently for each particular application and is not standardized. Even if it is attempted to standardize a truth table, this requires a number of modifications and much time. Two examples of such a conventional truth table are shown in FIGS. 8A and 8B. Symbols ROP, CBR, WT, SWT, RT, SRT, R/W, FW, and LC used in these truth tables indicate the following operation functions. Namely, ROP: Raster Operation, CBR: CAS Before RAS Refresh, WT: Write Transfer, SWT: Split Write Transfer, R/T: Read Transfer, SRT: Split Read Transfer, R/W: Normal Read/Write, FW: Flash Write, and LC: Load Color Register. The contents of these operation functions are not directly related to the gist of this invention, so the description therefor is omitted. Various operation functions correspond to various areas of a truth table. Each operation function in a truth table is determined in accordance with the states of external signals RAS, CAS, DT, WT and DSF, and there are sixteen types of operation functions which are determined by the states of CAS, DT, WE and DSF at the falling edge of RAS. The allocations of operation functions are different between the truth tables shown in FIGS. 8A and 8B so that circuit designs must be changed depending upon which truth table is used. Conventionally, after a truth table and specification are determined, circuit designs start for the manufacture of final devices.
Thus, a change in a truth table during or after circuit designs results in a change in circuits, which is difficult to carry out in the practical sense. It is therefore important to decide which truth table is to be used as the final target.